1. Field of Invention
The present invention generally relates to a multiple bit flash memory, and more particularly, to a reference current generation circuit for the multiple bit flash memory.
2. Description of Related Art
Flash memory is one kind of the non-volatile memory that performs multiple data program, read and erase operations. Since the data stored in it does not disappear due to the interruption of the power supply and the stored data is easy to be modified via the erase and program operation, it is widely used now in electronic equipment such as the personal computer.
The typical flash memory comprises a plurality of flash cells, and each flash cell generally stores a bit of data. The doped polycrystalline silicon is used to form a floating gate and a control gate of the flash memory. The floating gate and the control gate are isolated by a dielectric layer, and the floating gate and the substrate are isolated by a tunnel oxide. When the flash cells are performing the program/erase operation, a bias voltage is applied onto the control gate and drain to inject electrons into the floating gate or pull out electrons from the floating gate. When reading data from the flash cells, a word-line voltage is applied onto the control gate. The electric status of the floating gate impacts the ON/OFF status of the channel below it, and the ON/OFF status of the channel is used to determine whether the data is xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d.
In line with progress in semiconductor technology and the increase in the requirements for flash memory capacity, a multiple bit flash memory has been developed, and each flash cell stores a data that has more than two bits. Therefore, when reading the data stored in the flash cells, the current that is read has to compare with a reference current to determine the value of the stored data.
FIG. 1 schematically shows a sketch map of the threshold voltage distribution for the flash memory""s 2-bit flash cell. Where the abscissa represents the magnitude of the threshold voltage Vth, and the ordinate represents the quantity of each threshold voltage Vth""s flash cell. Moreover, the quantity distribution is like a gauss distribution as shown in the diagram. From FIG. 1, when erasing the flash cells, the threshold voltage Vth is below EV; when programming a xe2x80x9c01xe2x80x9d data value into the flash memory, the threshold voltage Vth is between PV1 and R2; when programming a xe2x80x9c10xe2x80x9d data value into the flash memory, the threshold voltage Vth is between PV2 and R3, and when programming an xe2x80x9c11xe2x80x9d data value into the flash memory, the threshold voltage Vth is above PV3. Therefore, the word-line voltage EV is applied onto the control gate when erasing the flash cells, and whether the erasing operation is completed or not is determined via the read current; when programming xe2x80x9c01xe2x80x9d, xe2x80x9c10xe2x80x9d, and xe2x80x9c11xe2x80x9d data values into the flash cells, the word-line voltage PV1, PV2, and PV3 are respectively applied onto the control gate, and whether the programming operation is completed or not is determined via the read current. When reading the data value stored in the flash cells, the word-line voltages R1, R2, and R3 are respectively applied onto the control gate, and the data value is determined via the read current. Moreover, the read current is compared with a reference current to determine the data value that is read.
The conventional reference current generation circuit used to achieve the objective mentioned above applies the boosted word-line voltages (BWLV) with different levels onto gates of different reference cells. When the 2-bit flash cell mentioned above is exemplified, since totally there are seven different levels for the word-line voltages for the erase verify, the program verify, and the read operations, they are EV, PV1, PV2, PV3, R1, R2, and R3, so that the boosted word-line voltages with seven different levels are needed for implementation. For example, the boosted word-line voltages with 15 different levels are needed to implement the 3-bit flash cell. Since each boosted word-line voltage has different variance when it is impacted by the variance of the temperature and the power voltage Vcc, each reference current generated by the reference generation circuit via this method has different drift along with the variance of the temperature and the power voltage Vcc.
Therefore, the present invention provides a reference current generation circuit, it can solve the problem of the reference currents having different drifts along with the variance of the temperature and the power voltage Vcc.
In order to achieve the objective mentioned above and others, the present invention provides a reference current generation circuit and it is suitable for a multiple bit flash memory. The reference current generation circuit comprises a plurality of reference current generation units, and each reference current generation unit comprises a load and a reference cell. The load comprises a first connection terminal and a second connection terminal, wherein the first connection terminal couples to an operating power, the second connection terminal couples to a first source/drain of the reference cell, the second source/drain is grounded, the gate of the reference cell couples to a boosted word-line voltage. The gate of each reference current generation unit couples to the same boosted word-line voltage, and the gate width of the reference cell for each reference current generation unit is different based on the magnitude of a reference current needed to be generated by the reference current generation circuit.
In a preferred embodiment of the present invention, the reference cell is a dummy cell, and the so-called dummy cell is formed by merging the floating gate and the control gate of the multiple bit flash memory""s flash cell having the same structure together. For ease of handling the reference cell characteristic, the dimension of the gate length of the reference cell for each reference current generation unit should be made the same, and the dimension of the gate length of the reference cell should be made greater than the dimension of the gate length of the multiple bit flash memory""s flash cell. For example, the dimension of the gate length of the reference cell should be designed as 1 xcexcm.
Moreover, in order to further improve the characteristics of the reference cells, the reference cells whose quantity are greater than the quantity needed by each reference current generation unit are formed in the same bank, and one or more than one reference cells are used to generate the reference current. When a plurality of the same reference cells in the same bank are used to generate the reference current, all the same reference cells used are tightly coupled in parallel to generate the reference current.
From the description mentioned above, the reference current generation circuit provided by the present invention applies the same boosted word-line voltage to the gates of different reference cells, and uses different gate lengths from different reference cells to obtain the reference currents with different levels that are needed. Therefore, it effectively solves the problem of the reference currents having different drifts along with the variance of the temperature and the power voltage Vcc.